Vol.  49.] 
THE  LLETX,  AXD  ASSOCIATED  VOLCAXIC  EOCES. 
153 
rraceable  over  the  whole,  but  interrupted  by  pseudocrystallites.  The 
iron  oxide  usually  is  concentrated  towards  the  centre,  and  sometimes 
the  outer  rim  is  clear. 
The  spherulitic  habit,  however,  is  not  confined  to  structures  which 
can  be  distinguished  in  hand-specimens.  In  very  many  slides, 
radial  clusters  of  felspars  are  more  or  less  regular  near  their  centre, 
but  have  no  definite  boundary,  adjacent  groups  being  confusedly 
inter  crystallized.  In  the  sreen  diabase  the  crystals  are  long,  nar- 
row,  and  straight.  In  the  basalt  they  are  narrow  and  long,  but 
generally  bent  or  curved,  often  undulating,  and  bordered  by  ferrite- 
granules  or  bars.  (PI.  I.  fig.  2.)  Even  in  some  of  the  defined 
spherulites  the  felspars  are  wavy,  lath-shaped  crystals  with  iron 
oxide  along  their  edges.  Possibly  this  is  the  cause  of  the  peculiar 
bending,  for,  if  iron  oxide  were  separated  in  any  great  quantity  at 
an  early  stage,  it  might  interfere  with  the  development  of  the 
felspars. 
Thus  the  spherulitic  growth  exhibits  roughly  three  or  four 
different  modes  1  : — 
1st,  without  radial  structure  ; 
2ndly,  with  sheaf-like  felspar  radiating  from  a  centre  and  giving 
a  black  cross  in  polarized  light — the  felspathic  type  : 
3rdly,  with  radiate  feathery  arrangement  of  iron  oxide,  which 
gives  the  reddened  colour  to  most  examples — the  ferruginous 
type. 
4thly,  with  grouped  arrangement  of  distinct  crystals.  Although 
in  the  spherulites  indications  of  these  may  be  found,  they  are 
best  shown  in  the  confused,  interlocking  aggregates  of  the 
groundmass. 
(b)  Distribution. — The  variolite  occurs  in  small  bosses  near  Aber- 
daron  and  at  places  aloug  the  coast;  it  is  well  seen  north  of  the 
beach  of  Porth  Oer,  near  the  small  isthmus  of  Dinas-fach,  and  in 
the  cliff  south  of  Porth  Orion. 
Towards  the  mouth  of  Afon  Sant,  near  Aberdaron,  the  diabase 
appears  to  form  two  or  more  patches,  suggesting  intrusion  amid  the 
ordinary  schistose  rocks.2  The  spherulites  are  only  inch  across, 
3  See  Quart.  Journ.  Geol.  Soc.  vol.  xli.  (1885)  Proc.  pp.  90-91,  Prof.  T.  G. 
Bonney's  Pres.  Address;  see  also  I.  P.  Iddings.  ‘  Spherulitic  Crystallization,’ 
Bull.  Phil.  Soc.  Washington,  voL  xi.  (1891)  p.  418. 
2  It  occurs  west  of  the  line  of  fault,  as  a  low  crag  overlooking  the  small  valley 
or  rift  along  the  line  of  the  streamlet,  and  it  crops  out  in  the  fields  above  in 
some  furze-covered  plots.  I  could  find  only  a  single  and  very  rotten  junction 
with  the  schistose  rooks,  and  at  that  part  the  diabase  seemed  to  be  compact  and 
without  spherulites.  The  variolitic  structure  appeared  not  to  be  necessarily 
developed  at  the  exterior  of  the  mass,  but  often  to  be  connected  with  a  kind 
of  lamination,  which  I  was  inclined  to  attribute  to  a  movement  in  the  magma. 
As  previously  stated,  in  one  part  at  least  the  rock  suggested  flow-brecciation, 
and  the  spherulites  are  usually  elliptical  -with  a  definite  orientation. 
In  the  black  micaceous  shales  quarried  south  of  the  fault,  east  of  the  stream, 
I  found  a  fragment  of  a  small  trilobite.  Dr.  Hicks  very  kindly  examined  this, 
and  stated  that  it  was  probably  the  pygidium  of  an  JZglina.  The  specimen,  in 
conjunction  with  others  from  another  locality,  seemed  to  him  suggestive  of  the 
Arenig  Group. 
